Sildenafil, widely recognised as the active ingredient in Viagra, is an effective treatment for erectile dysfunction, but understanding its disadvantages is essential before use. Whilst many men tolerate this phosphodiesterase type 5 (PDE5) inhibitor well, it carries potential side effects ranging from common headaches and facial flushing to serious cardiovascular risks and drug interactions. Certain medical conditions and concurrent medications make sildenafil unsuitable or require careful monitoring. This article examines the key disadvantages, contraindications, and safety considerations associated with sildenafil to help patients and healthcare professionals make informed treatment decisions.

What Is Sildenafil and How Does It Work?

Sildenafil is a medication primarily used to treat erectile dysfunction (ED) in men and, in some formulations, pulmonary arterial hypertension. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors. The medication works by enhancing the natural erectile response to sexual stimulation, rather than causing an erection directly.

The mechanism of action involves the inhibition of the PDE5 enzyme, which is responsible for breaking down cyclic guanosine monophosphate (cGMP) in the smooth muscle cells lining blood vessels. When sexual stimulation occurs, nitric oxide is released in the erectile tissue of the penis, triggering an increase in cGMP levels. This causes smooth muscle relaxation and increased blood flow to the corpus cavernosum, resulting in an erection. By blocking PDE5, sildenafil allows cGMP to accumulate, thereby prolonging and enhancing the erectile response.

Sildenafil is available in various strengths (typically 25mg, 50mg, and 100mg tablets) and is usually taken approximately 30-60 minutes before sexual activity. The effects can last for up to 4-5 hours, though this varies between individuals. In the UK, sildenafil is available both as a branded product (Viagra) and as generic formulations. Viagra Connect (50mg) is available as a pharmacy medicine without prescription, while other strengths remain prescription-only. For pulmonary hypertension, sildenafil is available under the brand name Revatio (typically 20mg three times daily), though these products are not interchangeable with ED formulations.

A high-fat meal can delay absorption and reduce the effect of sildenafil. The medication should not be taken more than once daily.

Whilst sildenafil has proven effective for many men with erectile dysfunction, it is important to understand that the medication is not suitable for everyone and carries potential disadvantages, side effects, and risks that require careful consideration before use.

Common Side Effects of Sildenafil

Like all medications, sildenafil can cause side effects, though not everyone experiences them. Understanding these common adverse effects is essential for patients considering or currently taking this medication. The majority of side effects are mild to moderate in severity and typically resolve without medical intervention.

The most frequently reported side effects include:

• Headaches – very common (affecting more than 1 in 10 people), these are often mild and related to the vasodilatory effects of the drug

• Facial flushing – common (affecting up to 1 in 10 people), a sensation of warmth and redness in the face and neck

• Indigestion or dyspepsia – common, this may include heartburn or stomach discomfort

• Nasal congestion – common, presenting as a blocked or stuffy nose

• Dizziness – common, some men experience light-headedness, particularly when standing up quickly

• Visual disturbances – common, these may include a blue tinge to vision, increased sensitivity to light, or blurred vision

These side effects are generally dose-dependent, meaning they may be more pronounced at higher doses. The visual changes occur because sildenafil has a mild inhibitory effect on PDE6, an enzyme found in the retina. Whilst usually temporary and harmless, any persistent visual changes should be reported to a healthcare professional.

Most common side effects diminish as the body adjusts to the medication or resolve within a few hours as the drug is metabolised. However, if side effects are persistent, troublesome, or interfere with quality of life, patients should consult their GP. Dose adjustment or alternative treatments may be considered.

Do not drive or operate machinery if you experience dizziness or visual disturbances. It is important never to exceed the prescribed dose in an attempt to improve efficacy, as this increases the risk of adverse effects without necessarily improving therapeutic benefit.

Suspected adverse reactions should be reported via the Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Serious Risks and Contraindications

Whilst most men tolerate sildenafil well, there are serious risks and contraindications that must be carefully considered. These disadvantages can be life-threatening in certain circumstances, making pre-treatment assessment essential.

Cardiovascular risks represent a significant concern. Sildenafil causes systemic vasodilation and can lead to a drop in blood pressure. Men with certain cardiovascular conditions should not use sildenafil, particularly those who have recently experienced a heart attack or stroke (within the past six months). The medication is contraindicated in patients with severe heart failure, unstable angina, or hypotension (blood pressure <90/50 mmHg). Sexual activity itself places demands on the cardiovascular system, and may be inadvisable for those with recent life-threatening arrhythmias or severe cardiovascular disease.

Priapism – a prolonged, painful erection lasting more than four hours – is a rare but serious adverse effect requiring emergency medical attention. If left untreated, priapism can cause permanent damage to penile tissue and result in irreversible erectile dysfunction. Patients with conditions predisposing them to priapism, such as sickle cell disease, multiple myeloma, or leukaemia, should use sildenafil with caution.

Non-arteritic anterior ischaemic optic neuropathy (NAION) is a rare condition causing sudden vision loss that has been reported in association with PDE5 inhibitor use. Men with a history of NAION in one eye have an increased risk and should discuss the benefits and risks with their doctor. If you experience sudden vision loss, stop taking sildenafil and seek immediate medical attention.

Absolute contraindications include:

• Concurrent use of nitrate medications (see next section)

• Concomitant use of riociguat (a soluble guanylate cyclase stimulator)

• Severe hepatic impairment

• Hypotension (blood pressure <90/50 mmHg)

Sildenafil is not recommended for patients with hereditary degenerative retinal disorders due to lack of safety data.

In severe renal impairment (creatinine clearance <30 mL/min), a reduced starting dose of 25mg should be considered.

If you experience chest pain after taking sildenafil, do not take nitrate medications (such as GTN spray or tablets). Call 999 immediately and inform the responders that you have recently taken sildenafil. Patients experiencing sudden vision or hearing loss, or an erection lasting more than four hours should also seek immediate medical attention.

Drug Interactions and Precautions with Sildenafil

Sildenafil has clinically significant interactions with several medications, and understanding these is crucial for patient safety. The most serious interaction occurs with nitrate medications, including glyceryl trinitrate (GTN), isosorbide mononitrate, and isosorbide dinitrate, which are commonly prescribed for angina. The combination of sildenafil with nitrates can cause a profound and potentially fatal drop in blood pressure. This contraindication is absolute, and patients must not take sildenafil if they are using nitrates in any form, including sublingual sprays or patches. Recreational use of amyl nitrite ('poppers') similarly poses serious risks.

Alpha-blockers, used to treat benign prostatic hyperplasia and hypertension (such as doxazosin, tamsulosin, and alfuzosin), can also interact with sildenafil, causing symptomatic hypotension. If concurrent use is necessary, patients should be stable on alpha-blocker therapy before starting sildenafil at a reduced dose of 25mg, with careful monitoring for postural hypotension. Consider separating the timing of doses to minimise this risk.

Other significant interactions include:

• CYP3A4 inhibitors – medications such as erythromycin, clarithromycin, ketoconazole, itraconazole, and ritonavir can increase sildenafil levels, necessitating dose reduction to 25mg

• Riociguat – concomitant use with this medication for pulmonary hypertension is contraindicated

• Other antihypertensive medications – whilst not contraindicated, blood pressure should be monitored as additive effects may occur

• Other PDE5 inhibitors – concurrent use with tadalafil or vardenafil is not recommended

Patients should inform their GP and pharmacist of all medications they are taking, including over-the-counter products and herbal supplements. Grapefruit juice can increase sildenafil levels and should be avoided. Excessive alcohol consumption may impair erectile function and increase the risk of side effects such as dizziness and hypotension.

Certain medical conditions require dose adjustment or additional monitoring, including moderate hepatic impairment, severe renal impairment (starting dose 25mg), anatomical penile abnormalities, and bleeding disorders. Patients over 65 years may also benefit from a lower starting dose. A thorough medical history and medication review are essential before initiating sildenafil therapy to minimise risks and optimise safety.

Frequently Asked Questions